1. Field Of The Invention
This invention relates to a spark plug for internal combustion engines and, more particularly, to a spark plug which is provided with heat pipe means to vary the heat range of the spark plug automatically.
2. Background Of The Invention
Spark plugs, particularly those in high-speed, high-compression engines, are subjected to an extreme range of pressure and temperature conditions. Plug temperatures range from about 200.degree. C. (392.degree. F.) at low engine speeds and light loads, to as high as 850.degree. C. (1562.degree. F.) under full throttle, full load. Below about 450.degree. C. (842.degree. F.), carbon and other products of combusting begin to form on the plug insulator nose. If not removed, those deposits build up until current shorts through the deposits instead of sparking across the electrodes. At normal speeds, enough heat is usually generated to burn those deposits away as quickly as they are formed. However, when high speeds or heavy loads raise the plug temperatures above 600.degree. C. (1112.degree. F.) to 700.degree. C. (1292.degree. F.), deposits not burned away, particularly those resulting from the additives in currently available fuels and lubricants, are melted to form a glaze coating on the plug insulator nose. When hot, this glaze is highly conductive and the plug is shorted out. This causes misfiring with consequent fuel and power losses. Should plug temperatures becomes excessive, the plug points themselves become hot enough to ignite the fuel-air mixture in the cylinder. This causes auto-ignition and, if continued, can lead to the destruction of the plug and serious engine damage. Overheated spark plug electrodes also cause a condition commonly met in two-stroke engines: the bridging of the electrodes due to the build-up of conducting deposits formed by combustion particles which have melted upon their striking the overheated electrodes. In plug temperature ranges above 850.degree. C., chemical corrosion and spark erosion cause plug failure within a very short time.
It will be seen then, if a hot-type plug is subjected to high compression pressures, temperatures, and loads, electrode buring and auto-ignition will result because of the plug's slow rate of heat transfer. A cold plug, because it will not reach full operating temperature, will not tolerate low-speed, light-load operation for any length of time without becoming fouled with current-conducting deposits. Because a cold plug under such conditions will not reach a temperature required to burn off fouling, carbon formation as well as additive particles from the fuel and oil condensing on the comparatively cool surfaces of the insulator will foul the plug and will cause it to misfire.
Spark plugs are customarily supplied in various heat ranges to handle the requirements of individual engines and operating conditions. Heat range refers to the ability of the plug to conduct the heat of combustion away from the electrodes or firing end. As shown in FIG. 1, a conventional hot-type plug will have a long insulator nose 2. Because of the length of the heat path (as indicated by the arrows (3), heat thus will be transferred comparatively slowly from the plug firing end to the engine cooling system. A conventional cold-type plug (FIG. 2), on the other hand, has a comparatively short insulator nose 4 and heat is transferred rapidly (as indicated by the arrows 5) in to the engine's cooling system.
3. Description Of The Prior Art
The prior art discloses several examples of spark plugs incorporating means which are intended to vary the heat range thereof automatically such that the device can accommodate a wider than normal spectrum of operating conditions. In one such example in the prior art disclosed by P. G. Andres in U.S. Pat. No. 2,212,725, a skirt having segments of bimetallic material is positioned on the ceramic insulator with a gap therebetween in the cold condition. As the plug firing end heats up in operation, the segments contract to close the gap such that heat travels up the skirt to expedite the transfer of heat from the firing tip. Inasmuch as the conductance of heat between two bodies such as between the skirt and the insulator is dependent upon the establishment of a good thermal contact, and the design in the cited Andres patent is such that any appreciable degree of plug fouling will adversely effect the thermal contact, the performance characteristics of the device is likely to be erratic and difficult to maintain under the expected operating conditions. In another such spark plug disclosed by H. W. Andersen in U.S. Pat. No. 3,130,338, a similar segmented skirt of bimetallic material is fitted on the ceramic insulator in the area above the lower insulator gasket of the spark plug. Although that Andersen design will have some effect on the overall spark plug temperatures, it does not appear that it would effect to any approciable extent the temperature of the nose or firing end of the spark plug which is the area of concern of the instant invention. The inventor in the present invention has disclosed multiple heat-range spark plugs in U.S. Pat. Nos. 3,612,931 and 3,743,877. In those prior art designs, a heat shunt is provided on the firing tip of the spark plug, said shunt moving into contact with the shell of the spark plug at the design temperature to maintain the firing tip at an optimum operating temperature. In those prior art designs, the heat shunts are fabricated out of metal; this factor implies a certain restriction on the freedom of design.
In the prior art, A. A. Kasarjian, in U.S. Pat. No. 2,096,250, discloses a spark plug that has a hollow longitudinal space in the center electrode extending substantially the length thereof, the space being nearly filled with a material possessing high heat conductivity. In the design of Kasarjian, the conductivity of the said material absorbs heat from the firing tip and carries it to the cooler parts of the spark plug, by convection as well as by conductance, and dissipates it there. Inasmuch as the longitudinal space in the center electrode is filled with the cooling medium with the exception of a small void to compensate for the thermal expansion of the medium, it will be seen that the cooling means in the spark plug of kasarjian does not use the heat pipe principle involved in the operation of the subject invention.